An electric hand tool drives an application tool to operate using rotational energy generated from an electric motor it includes. Taking a grinding machine tool for example, the electric motor drives a grinding disc to rotate when activated to further perform a grinding operation. However, when the electric motor rotates for a certain period of time and is controlled to stop operating, even if the electric motor does not continue receiving the driving signal, a rotor of the electric motor and the grinding disc cannot immediately stop rotating due to the inertia stored during the rotation of the grinding disc, and continue to rotate for a short period of time.
A technical solution is provided in view of the above issue. For example, the Taiwan Patent No. M349819 discloses a decelerating device for a grinding disc of a grinding machine, and more particularly a grinding disc structure for decelerating a rotational speed of a grinding machine. The above grinding disc structure includes a grinding disc, a decelerating device and a body. The grinding disc includes a fixing portion for fixing purposes. An outer edge of the fixing portion forms a contact plane, and a grinding plane for performing a grinding process is formed at the other surface opposite the contact plane. The decelerating device is abutted against the contact plane of the grinding disc, and is provided with a damping body for decelerating the speed of the grinding disc. The damping body may be accommodated in an inner chamber provided in a recessed manner at a periphery of a base. An axial opening is provided at a center position of the base. A sliding groove for accommodating a driving body is provided at an outer peripheral surface of the axial opening. An embedded limiting opening is provided at a center of the driving body, which is covered by a cover plate incorporable with the base. A through hole facing the axial opening is provided a center position of the cover plate, and a sliding channel for accommodating the driving body is provided in a recessed manner at an outer periphery of the through hole. The grinding machine allows the grinding disc and the decelerating device to be assembled and combined. The body includes an internal accommodating space, above which the decelerating device provides a shield using the base. The accommodating space is internally provided with a driving shaft, which allows the fixing portion and the axial opening to be assembled and fixed to perform driving operations.
However, due to constant contact between the damping body and the contact plane in the above structure, the rotational speed of the grinding disc is reduced during the rotation process, such that the fineness and quality of grinding are degraded. Further, such technology also adds a burden to the electric motor and is unbeneficial for applications of the electric motor. Moreover, wear is caused by abrasion between the damping body and the contact plane over an extended period of time, and replacement of these components is needed on a regular time basis. Further, certain amount of powder is produced at a surface of an object being grinded due to the abrasion of the damping body and the contact plane to result in pollution.